At present, the causes of gastrointestinal symptoms are not completely understood (for a summary of symptoms and their prevalence in athletes see this previous blog). The symptoms are difficult to investigate because they are often specific to race situations and are very difficult to reproduce or simulate in a laboratory. Nevertheless, some laboratory studies have been performed, and field studies have correlated the symptoms with nutritional intake and other factors.
From these studies a number of potential causes and contributors have been identified and they can be divided into 3 general categories:
(2) mechanical, and
Physiological causes of GI-symptoms include reduced blood flow and increased anxiety (especially before competition). With exercise, blood flow is preferentially redirected to the working muscles and blood flow to the gut can be reduced by as much as 80%. Such low blood supply can compromise gut function to varying degrees and can result in commonly experienced GI symptoms such as cramping. In severe cases it can even result in injury of the large intestine as a result of inadequate blood supply (ischemic colitis). Although, with training, this decrease in blood flow becomes less pronounced, there is no clear evidence that less fit individuals are more prone to developing symptoms as a results of reduced blood flow to the gut. Anxiety has an effect on hormone secretion which in turn can affect gut movement, resulting in incomplete absorption and loose stool.
The mechanical causes of GI-problems are either impact-related or are related to posture. Gastrointestinal bleeding is common amongst runners. This is thought to be a result of the repetitive high-impact mechanics of running and subsequent damage to the intestinal walls. This repetitive bouncing of the stomach during running is also thought to contribute to lower GI symptoms such as flatulence, diarrhea and urgency. Estimates of the incidence of occult blood (blood in feces) after a race range from 8% to 85% mostly because of the wide range of race distances in various studies. The longer the distance, the greater the incidence. As many as 16% of runners in one study report having bloody diarrhea on at least one occasion after a race or hard run. The mechanical trauma suffered by the gut from the repetitive bouncing of running in combination with reduced blood flow to the intestine (and oxygen deprivation of some parts of the intestine) are probably the cause of the bleeding. Presence of bloody bowel movements after an endurance event raise the possibility of severe health risks.
Posture can also have an effect on GI symptoms. For example, on a bicycle (especially in aero position), upper GI symptoms are more prevalent possibly due to increased pressure on the abdomen as a result of the cycling position, specifically when in the aero position. "Swallowing" air as a result of increased respiration and drinking from water bottles can result in mild to moderate stomach distress.
Finally, nutrition can have a strong influence on gastro-intestinal distress. Fiber, fat, protein, and fructose have all been associated with a greater risk to develop GI-symptoms. Dehydration, possibly as a result of inadequate fluid intake, may also exacerbate the symptoms. A study in Belgium demonstrated a link between nutritional practices and gastrointestinal complaints during a half–Ironman distance triathlon. Gastrointestinal problems were more likely to occur with the ingestion of fiber, fat, protein, and concentrated carbohydrate solutions during the triathlon. Beverages with high density (osmolarities >500 mOsm/L) seemed especially to be responsible for some of the reported complaints. (typically these osmolalities can be found in drinks that contain >12 g/100 ml of carbohydrate as well as electrolytes; most sports drinks are around 6 g/100 ml).
The intake of dairy products may also be linked to the occurrence of gastrointestinal distress. Mild lactose intolerance is fairly common and could result in increased bowel activity and mild diarrhea. Although some risk factors have been identified it is still unclear why some individuals seem to be more prone to develop GI-problems than others. To minimize gastrointestinal distress, all these risk factors must be taken into account, and milk products, fiber, high fat, and high protein must be avoided 24 hours before competition and during exercise. A more detailed discussion of preventative measures will follow in the next blog.
de Oliveira, E. P., Burini, R. C., Jeukendrup, A. 2014. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med 44 Suppl 1: S79-85.
The full text of this review article can be downloaded here and is free (Open Access)